1. Field of the Invention
The invention relates to a control system for an internal combustion engine that uses compressed natural gas (CNG) as fuel and an inert gas concentration detecting device for CNG.
2. Description of Related Art
Japanese Patent Application Publication No. 2004-346911 (JP 2004-346911 A) describes a technique for correcting a fuel injection amount on the basis of a fuel property in an internal combustion engine that uses CNG as fuel. In JP 2004-346911 A, a fuel property is identified on the basis of a measured value of an in-cylinder pressure sensor that measures a pressure in a cylinder. A fuel injection amount is corrected on the basis of the identified fuel property.
Japanese Patent Application Publication No. 2004-346842 (JP 2004-346842 A) describes a technique relating to start control in a bifuel engine that is able to independently supply gasoline and CNG as fuel.
The properties of CNG that is used as fuel in the internal combustion engine are not always uniform. That is, the amount of inert gas (for example, carbon dioxide (CO2) and nitrogen (N2)) contained in CNG may significantly vary depending on, for example, a CNG refueling station. As the inert gas concentration of CNG that is used as fuel in the internal combustion engine varies, the combustion state of the CNG may be instable or the exhaust characteristic may deteriorate.